1. Field of the Invention
The present invention relates generally to an electrochemical cell to be used for a high reliability application such as an implantable medical device. The cell of the present invention is particularly useful in an implantable medical device such as a cardioverter defibrillator or pacemaker. In such an application, it is critical to be able to provide a clear indication that the cell is approaching end-of-life. The cell can thus be replaced prior to its being unable to adequately power the device, which could result in loss of device function. More particularly, the present invention relates in one embodiment to a method for selecting the optimal replacement voltage of an electrochemical cell.
2. Description of Related Art
A currently preferred power source for an implantable medical device is an alkali metal electrochemical cell, such as of lithium coupled with a sandwich cathode. The sandwich cathode design comprises a second cathode active material of a relatively high energy density but of a relatively low rate capability sandwiched between two current collectors with a first cathode active material having a relatively low energy density but of a relatively high rate capability in contact with the opposite sides of the current collectors.
A number of patents and publications have disclosed such electrochemical cells. For example, U.S. Pat. No. 6,551,747 to Gan, which is assigned to the assignee of the present invention and incorporated herein by reference, describes a sandwich cathode design having a second cathode active material of a relatively high energy density but of a relatively low rate capability sandwiched between two current collectors and with a first cathode active material having a relatively low energy density but of a relatively high rate capability in contact with the opposite sides of the two current collectors. A preferred anode is lithium (Li), a preferred first cathode material is silver vanadium oxide (SVO) and a preferred second cathode material is carbon monofluoride (CFx).
U.S. Pat. No. 6,926,991 to Gan et al., which is assigned to the assignee of the present invention and incorporated herein by reference, discloses a cathode design having a first cathode active material of a relatively low energy density but of a relatively high rate capability contacted to a first cathode current collector and a second cathode active material having a relatively high energy density but of a relatively low rate capability in contact with a second cathode current collector. The first and second cathode current collectors are connected to a common terminal lead. The preferred cell materials are also Li/SVO/CFx.
The present invention provides an early warning indicator as to when the cell's discharge capacity is nearing end-of-life (EOL) based on empirical observations of the discharge efficiency of the first and second cathode active materials. This early warning is defined as the elective replacement indicator (ERI) and signals a physician when it is time to replace the medical device. Suitable medical devices include cardiac defibrillators, neurostimulators, pacemakers, and the like.
Because an implantable electrochemical cell often supports such a life-sustaining device in a patient, it is critical to be able to identify the point at which the cell needs to be replaced before it is no longer functional. This gives the physician and patient time to replace the battery without jeopardizing the therapeutic function of the device. Typically, the replacement indicator is based on the potential of the cell under some defined load.
Historically, the Li/SVO cell system has been used as the power source for implantable cardiac defibrillator applications requiring high rate pulse capability, i.e., about 1 to about 4 amps. Since Li/SVO cells have a staged discharge voltage profile, a pre-determined background voltage is generally used as the ERI. This pre-determined voltage value varies depending on the cell size, theoretical capacity and the associated device design. Additionally, due to the characteristic voltage delay and growth in resistance under direct current (Rdc) that occurs at about the 2.6-volt plateau, a pre-determined Rdc or voltage value under high current pulsing is sometimes used as an ERI. Consequently, the ERI selection has heretofore been complicated and dependent on the individual device design of each cell manufacturer.
In general, the elective replacement indicator voltage should be chosen near the end of the cell's useable capacity, in order to maximize the lifetime of the cell. In one commonly used method of determining cell replacement indicating voltage, the capacity of the cell that is useable by the device is calculated and then the capacity required to give an adequate time for replacement is subtracted. The remaining cell capacity is considered the capacity available prior to replacement and can be correlated with a replacement voltage under a given load. Since the voltage of the cell is load dependent, this methodology can be problematic. Under a heavy load, the replacement indicating voltage will be reached sooner, resulting in a reduction in apparent device lifetime. Alternatively, under a light load, the replacement indicating voltage will be reached later. This results in a reduction in time for replacement prior to loss of device function, and higher risk to the patient.
U.S. Pat. No. 6,926,991 to Gan et al., which is assigned to the assignee of the present invention and incorporated herein by reference, describes a method for providing a physician with an elective replacement indicator for an implantable medical device. The medical device is powered by an electrochemical cell having a lithium anode coupled to a sandwich cathode comprising the configuration SVO/current collector/CFx, with the SVO facing the anode. The indicator is predicated on when the cell's discharge capacity is nearing end-of-life (EOL) based on the theoretical capacity and the discharge efficiency of the SVO and CFx active materials, which serves as an indicator of when it is time to replace the medical device. Gan et al. provide a mechanism for determining both EOL and ERI by varying the relative weight of SVO to CFx in a cathode having one of the following configurations: SVO/current collector/CFx/current collector/SVO, SVO/current collector/SVO/CFx/SVO/current collector/SVO, SVO/current collector/CFx with the SVO facing the anode, and SVO/current collector/SVO/CFx with the SVO facing the anode.
In use with a device to be powered, a cell may be subject to numerous transient perturbations such as a series of high current pulses. After a perturbation, the cell voltage will be artificially lowered until concentration gradients within the cell have relaxed. If the cell voltage is sampled too soon after a perturbation, the replacement indicating voltage could be prematurely indicated, reducing the lifetime of the cell. Alternatively, if the device is programmed to wait an excessive amount of time after a perturbation to begin monitoring voltage relative to the replacement indicator, the replacement indicating voltage could be indicated late, resulting in too short a time for cell replacement prior to loss of device function.